It is frequently necessary to seal a clearance gap between two components that are capable of relative movement. In particular one or more seals are often required to provide a seal between a rotatable shaft and an axially adjacent static component. For example, a gas turbine engine comprises shafts which rotate at relatively high speeds and which are exposed to pressurised hot gases. Seals are required between rotating rotor blades and surrounding static casing structure.
Seals are also required between a rotor carrying such rotor blades and an adjacent static structure which carries stator vanes or nozzle guide vanes. In a gas turbine engine nozzle guide vanes or stator vanes are non-rotating and as such mounted on a static structure.
It is also important to provide such seals to prevent hot pressurised gas in one pressure zone flowing freely into an adjacent lower pressure zone.
It is well known to provide labyrinth seals to seal between rotating and non-rotating members, however large clearances are required at some engine operating conditions to accommodate relative movement. It is desirable to reduce or alter these clearances at certain operating conditions to achieve the most effective power output of the engine.
It is also a requirement of such seals that an acceptable clearance gap is provided at all the differing engine conditions. Such seals between rotating and non-rotating members are not required to provide a complete closed seal but are required to provide a seal with a predetermined clearance range.
Thermal expansion and changes in pressure conditions can also cause unbalanced forces on the seal and affect the seals effectiveness. It is therefore important that the clearance between the non-rotating part of the seal and its static part is kept within a predetermined range.